The conversion of synthesis gas, i.e. carbon monoxide and hydrogen, to higher value products is well known and has been in commercial use for many years. Typical processes include, for example, methanol syntheses, higher alcohol synthesis, hydroformylation and Fischer-Tropsch synthesis. The synthesis gas mixture is contacted with a suitable catalyst typically comprising at least one Group VIII metals. Suitable Fischer-Tropsch catalysts comprise one or more catalytic Group VIII metals, such as iron, cobalt and nickel. For oxygenate synthesis, copper may be included as well.
There exist many variations of the formulation and preparation of catalysts useful for the conversion of synthesis gas. In general, the catalysts are classified into two broad types, unsupported metals, known as Dispersed Active Metals and a larger groups of catalysts metals supported on refractory oxides, such as silica, alumina, titania or mixtures thereof. Such catalysts, whether supported or unsupported may be enhanced by the addition of other metals or metal oxides, known as promoter metals.
Supports for catalyst metals are generally pilled, pelleted, beaded, extruded, spray-dried or sieved materials. There are many methodologies reported in the literature for the preparation of supported catalyst metals. Examples of such techniques include incipient wetness impregnation, slurry impregnation, coprecipitation, and the like. It will be appreciated that high metal loadings are generally obtained by coprecipitation or multiple, i.e. two or three, impregnations, whereas low metal loading catalysts may be prepared utilizing a single impregnation. The catalyst metal content of such catalysts may vary from one to fifty weight percent. Promoter metals or metal oxides may be added during the impregnation steps using soluble salts of the respective metals such as Pt, Pd, Rh, Ru, Os, Ir, Mo, W, Cu, Si, Cr, Ti, Mg, Mn, Zr, Hf, Al, Th and the like. It will further be appreciated that the choice of a particular metal combination and the amount thereof to be utilized will depend upon the specific application used in the conversion of synthesis gas. When a suitable support has been impregnated with one or more metals as by impregnation to form a catalyst precursor, it may be dried and then calcined in an oxygen-containing environment. The precursor is thereafter activated by reduction at elevated temperature in the presence of a reducing gas, typically containing hydrogen. Optionally, the catalyst is activated by contacting with hydrogen gas in presence of liquid hydrocarbons as disclosed in U.S. Pat. No. 5,292,705.
Regardless of the particular formulation and method of preparation, all catalysts lose productivity and/or selectivity in use. Selectivity may vary with the particular synthesis, but is generally expressed in terms of the percent of an undesirable substance in the product mix. For example, methane selectivity in a Fischer-Tropsch reaction is the percent of methane formed with the desired higher hydrocarbons. Degradation of the catalyst productivity may be due to a number of phenomena including, without limitation, contamination by catalytic poisons, deposition of carbonaceous residues, sintering, phase transformation of the metal or metals and the like. U.S. Pat. No. 5,283,216 discloses a method for rejuvenating an hydrocarbon synthesis catalyst, which has been subjected to reversible, partial deactivation in a slurry synthesis process by contacting the catalyst with hydrogen at elevated temperatures in presence of liquid hydrocarbons. However, not all deactivated catalysts are rejuvenable. It is commercially significant to extend the useful life of a used catalyst by various treatment procedures, for example, by means of regeneration.
There are catalyst regeneration methods described in the literature. Typically, these techniques rely on contacting the used catalyst at elevated temperature with an oxygen-containing gas and/or steam. Such treatment may be used to remove carbonaceous deposits and poisons additionally converting the metal to its corresponding oxide or oxides. The regenerated catalyst is thereafter reactivated by means of a reduction with a hydrogen-containing gas at elevated temperatures. Such a treatment is described, for example, in U.S. Pat. No. 4,399,234.
U.S. Pat. No. 2,369,956 discloses a method for regeneration of a Fischer-Tropsch catalyst wherein the catalyst is dissolved and subsequently restored by re-precipitation of the catalytic metals. It was noted, however, that there were deposits remaining in the contact substance that materially increased the difficulty of restoring the catalyst. An example of such substances is the high molecular weight paraffins from the used catalyst that make it difficult to filter the metal salt produced by dissolution of the catalyst with acid. Since these materials make purification of the salt difficult, it is taught in the patent that hydrocarbon deposits on the catalyst must be initially removed by treatment with flowing hydrogen at elevated temperatures. The process of dissolution and re-precipitation may then be carried out. It is also taught in the patent that the pyrophoricity of the treated catalyst might be mitigated by treatment with steam prior to dissolution with strong acid. However, there is nothing in the patent regarding the efficiency of the disclosed process, or the effect of exposing a catalyst support, such as described above, with strong acid.
U.S. Pat. No. 3,256,205 discloses a method of catalyst regeneration by treatment with a strong acid to the point of incipient wetness of the catalyst prior to removal of carbonaceous deposits accumulated during the catalytic cycle. It is specifically stated that removal of the carbonaceous deposits is detrimental in that the catalyst support would be damaged by contact with the strong acid utilized. Suitable acids are stated as having a dissociation constant greater that 10xe2x88x922 and are added to the catalyst in an amount varying from 0.5 stoichiometry to the stochiometry required to form the salts of the metals present in the catalyst.
Khodakov et al. In a paper in Oil and Gas Science and Technology Rev. IFP, 54, 525 (1999) teach that contacting a reduced cobalt catalyst with water, followed by drying and calcining in air results in the formation of smaller cobalt oxide crystallites relative to those that would be formed by decomposition of the initial cobalt salts. There is neither teaching nor suggestion that the disclosed methodology might have any application to catalyst regeneration.
It is clear from the foregoing discussion that there is not a clear incentive in the art to utilize any particular methodology in attempting to improve on the process of catalyst regeneration. In fact, the two patents discussed above would appear to negate each other since the first teaches that it is necessary to remove the carbonaceous deposits from the catalyst prior to treatment with acid, yet the second teaches that the carbonaceous deposits are necessary to prevent the acid from attacking the support structure. It also must be considered that it is generally not possible to use an aqueous-based solvent on a catalyst containing a waxy hydrocarbon deposit because it is hydrophobic as typically observed with Fischer-Tropsch catalysts. Hence, it would appear that the process of the second patent would not have applicability to a Fischer-Tropsch catalyst since a characteristic of the process is that the pores of the used catalyst are filled with wax that prevents good wetting by aqueous treatment solutions.
In hydroprocessing and oxidation catalysts, carbonaceous deposits are typically removed by calcination with an oxygen-containing gas at elevated temperatures. During such treatments, the metal-containing active phase of the catalyst is converted to oxides. To further improve the recovery of catalytic activity, contaminating metals are then removed by treatment with a basic solution, particularly one containing ammonium carbonate or sodium cyanide. Such treatments are illustrated, for example, in U.S. Pat. No. 4,795,726 and German Patent DE 43 02 992.
The modifying of hydroprocessing catalysts is taught, for example, in U.S. Pat. No. 5,438,028 wherein a finished catalyst is enhanced by the addition of a modifying agent in solution after which the catalyst is dried and optionally heated to a temperature of from 120xc2x0 C. to about 1000xc2x0 C. The process does not include a final reduction step to reactivate the catalyst. The modifiers disclosed in column three, with the exception of boron, which is not a metallic element, are all recognized poisons for Fischer-Tropsch catalysts. U.S. Pat. No. 5,389,502 discloses application of the same process for the enhancing of a hydroprocessing catalyst that has been regenerated by an oxidative treatment. The application of the modifying agent to the surface of the catalyst may be carried out to the point of incipient wetness. In both of these patents, the preferred modifying agent is boron.
U.S. Pat. No. 6,201,030 discloses a process and apparatus for regenerating a particulate catalyst during operation of a reactor. The process consists of withdrawing a partially spent catalyst as a slurry from a reactor to one of two regeneration stations, operating in parallel, treating the slurry with hydrogen and returning it to the reactor. The two regenerating stations are utilized in the alternative operating out of phase thereby facilitating continuous withdrawal and return of the slurry without substantial change in the liquid level within the reactor. The disclosed process effectively fails to provide any means of regenerating severely deactivated catalyst or of improving process reliability, such as by removing fines that may have formed in the turbulent environment of the reactor.
It is generally recognized that the economic worth of a given catalyst is a function of its original cost, its activity its regenerability and its value as a used catalyst, e.g. for metals recovery. It is apparent from the foregoing discussion that there has been considerable effort going back over many years to improve the economic worth of catalysts, since a process that will effectively increase the value of a catalyst and/or extend the useful life thereof before it must be disposed of through conventional metal recovery will significantly improve the worth of that catalyst. Effective catalyst regeneration effected while at the same time maintaining the reliability of the process requires the use of specific apparatus or combinations of specialized pieces of apparatus in combination with specific treatment techniques. Such process techniques and apparatus for carrying them out are provided in accordance with the present invention.
In accordance with the present invention, there is provided a significant improvement in the catalytic hydrogenation of carbon monoxide to form a mixture of hydrocarbons wherein the catalyst is a supported Fischer-Tropsch metal catalyst. The useful life of such catalysts is extended by a process of regenerating spent catalyst comprising: decreasing the hydrocarbon content of the catalyst, impregnating the catalyst with a solution of a least one metal salt, optionally in combination with at least one of an ammonium salt, alkyl ammonium salt, ammonia or a weak organic acid in the presence of a non-oxidative atmosphere, oxidizing the catalyst in the presence of the impregnating solution at low temperatures and reducing with a hydrogen-containing gas at elevated temperatures to form an active catalyst.
The catalyst treated according to the invention is advantageously reused for the hydrogenation of carbon monoxide. Optionally, the catalyst is withdrawn from a carbon monoxide hydrogenation reactor and returned to at least one reactor, preferably during operation of the reactors. One up to all of the treating steps through activation of the catalyst may be carried out prior to withdraw, subsequent to return, or between withdraw and return. The withdrawal and return steps may be carried out periodically or continuously.